Effect of phosphorus deficiency on phenolics and antioxidants content of two African nightshade varieties grown in Kenya

Indigenous vegetables form an integral part of the Kenyan diets, most commonly consumed being the African nightshade. These vegetables contain important phenolics and antioxidants that have medicinal and good health attributes. Their production has strongly been associated with environmental stresses, and phosphorus as one the limiting nutrients had been suspected to play key role. To investigate the effect of phosphorus stress on Total Phenolic Content (TPC) and Total Antioxidants Activity (TAA) on nightshade, greenhouse and field experiments were conducted. Two commonly grown varieties (Solanum villosum-SV and Solanum scabrum-SS) were planted in both conditions; done under long (May-July 2014) and short rain seasons (August-October 2014). It was laid as Randomized Complete Block Design with split plot arrangement. The two varieties were the main plot and phosphorus levels (0, 20, 40 and 60 kg/ha) constituted the subplot with four replicates. Gallic Acid (standard) Extraction method was used to analysis TPC. Diphenyl picryl hydrazyl method was used to analyze TAA where Vitamin C was standard. Data on TPC and TAA were recorded and later the effects resulting from these treatments analyzed using ANOVA and mean separation using Least Significant Difference (LSD) at p≤0.05. The TPC and TAA were significantly affected by different phosphorus levels (p≤0.05). TPC and TAA decreased with increase in phosphorus. SV had higher TPC and TAA (6.09 mg/g and 38.58% respectively) as compared to SS that had 5.49 mg/g (TPC) and 35.92% (TAA). SV had more phenolics and antioxidants in the shoots than roots, the converse was found for SS. Both varieties at 40 kgP/ha offered the best tradeoff between yield and secondary metabolites (phenolics and antioxidants). Study recommends 40 kgP/ha as it had the highest levels of phenolics and antioxidants. Further research needs to be done on other important antioxidants like anthraquinones and how different levels of macronutrients affect their production.